The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
There is much current interest in the development of materials or systems for adsorbing gas molecules, particularly for the purposes of gas storage or separation.
Hydrogen and methane are seen as the energy carriers of the future. Hydrogen as a combustion fuel is very environmentally friendly, generating only water as a combustion byproduct. Hydrogen is also an important fuel for fuel cells which generate electricity by the electrochemical oxidation of hydrogen. The use of adsorbed natural gas (ANG) which is primarily methane, as a vehicular fuel is seen as an attractive alternative to compressed natural gas (CNG), which requires operating pressures of 340 atm. so that sufficient gas can be stored on-board, thereby demanding complex multi-stage compression equipment.
However, the storage of hydrogen and methane in a safe and practical manner presents a formidable engineering challenge. Their efficient use as fuels in vehicular transportation is limited by the current requirement to store them in large, heavy and dangerous high-pressure or cryogenic tanks. Storage of hydrogen and methane for such applications is complicated by the fact that these gases are flammable and in some situations explosive. Alternative methodology for storage of these gases exists, but each of the current alternatives is undesirable for one or more reasons.
Carbon dioxide capture and storage is another current area of significant interest. Removal of carbon dioxide from the flue exhaust of power plants, currently a major source of anthropogenic carbon dioxide, is commonly accomplished by chilling and pressurizing the exhaust or by passing the fumes through a fluidized bed of aqueous amine solution, both of which are costly and inefficient. Other methods based on chemisorption of carbon dioxide on oxide surfaces or adsorption within porous silicates, carbon, and membranes have been pursued as means for carbon dioxide uptake. However, in order for an effective adsorption medium to have long term viability in carbon dioxide removal it should combine two features: (i) a periodic structure for which carbon dioxide uptake and release is fully reversible, and (ii) a flexibility with which chemical functionalization and molecular level fine-tuning can be achieved for optimized uptake capacities.
Current research into high volume storage of gases such as hydrogen has largely focussed on physisorption or chemisorption based materials. Metal-organic frameworks have shown great promise as materials with high gas adsorption capacity. They possess intrinsically high surface areas and internal volumes—factors useful for gas storage by physisorption at high pressures and/or low temperatures. However, these operating conditions require heavy and potentially expensive system components for implementation within hydrogen or methane powered vehicles. Consequently, materials that operate at near-to-ambient conditions are highly sought after, as the systemic requirement would be drastically reduced. In order to achieve operation under these conditions, the gas adsorption heat must be drastically increased.
Whilst increasing the heat of adsorption for physisorption based materials is crucial to their widespread implementation, chemisorption based materials such as magnesium and lithium metal hydrides have adsorption heats well above 15.1 kJ/mol, calculated as the value required for room temperature hydrogen storage. Consequently these materials require several hundred degrees for operation, a substantial energy cost.
In order for physisorbed methane (ANG) to present a realistic alternative to CNG for powering vehicles, the US Department of Energy has stipulated methane adsorption of 180 v/v at 298 K and 35 atm. as the benchmark for ANG technology, and the optimum adsorption heat has been calculated at 18.8 kJ mol. Most of the effort has been in the development of porous carbons as storage materials, however, even the most sophisticated carbons strain to obtain any significant improvements over the 180 v/v target, largely because of the inherently low adsorption heat of methane within carbons, typically 3-5 kJ/mol.
It would therefore be desirable to provide an alternative gas absorption material.